Conventionally, a solar cell of the substrate type has been known, which includes a substrate, a lower electrode formed on the substrate, a light-absorption layer formed on a conductive film, a window layer formed on the light-absorption layer, and an upper electrode formed on the window layer. Further, a solar cell of the substrate type that further includes a buffer layer formed between the light-absorption layer and the window layer has been known.
Regarding the conventional solar cell of the substrate type, more specifically, a configuration has been proposed that includes a glass substrate containing an alkali metal such as Na, a metal film (lower electrode) such as a MO film formed on the glass substrate by sputtering or the like, a chalcopyrite-structured compound semiconductor layer (light-absorption layer) having a p-type conductivity such as a p-type Cu(In,Ga)Se2 layer, which is formed on the metal film by multisource vapor deposition or the like, a CdS layer (window layer) formed on the compound semiconductor layer by a solution method, and a n-type transparent conductive film (upper electrode) such as a ZnO:Al film (see, for instance, JP10(1998)-74967A). To produce a solar cell with a high energy conversion efficiency, in the conventional configuration, the p-type Cu(In,Ga)Se2 layer functioning as the light-absorption layer was formed by applying a method in which a p-type Cu(In,Ga)Se2 crystal was grown slowly over a long time. This is because the slow crystal growth allows not only for the reduction of crystal defects in the p-type Cu(In,Ga)Se2 layer, but also the enhancement of the flatness of the surface thereof even though the layer is polycrystalline. Further, by forming a CdS layer on the p-type Cu(In,Ga)Se2 layer with the flat surface, the CdS layer can be formed with excellent coverage.
Further, another example of the conventional substrate-type solar cell has been proposed that includes a glass substrate, a metal film (lower electrode) such as a MO film formed on the glass substrate by sputtering or the like, a chalcopyrite-structured compound semiconductor layer (light-absorption layer) having a p-type conductivity such as a p-type Cu(In,Ga)Se2 layer, which is formed on the metal film by a selenidation method, a buffer layer such as ZnO film formed on the compound semiconductor, a window layer such as a ZnO:Al film, and an upper electrode (see, for example, JP10(1998)-135498A). In the selenidation method, the p-type Cu(In,Ga)Se2 layer may be formed by forming a CuGa/In/Se precursor (a stacked film composed of a CuGa film, an In film, and a Se film) and thereafter causing solid phase diffusion of the CuGa/In/Se precursor by heating, or alternatively, the p-type Cu(In,Ga)Se2 layer may be formed by forming a CuGa/In precursor and thereafter subjecting the same to heat treatment in H2Se gas.
Further, still another example of the conventional substrate-type solar cell has been known that includes, as a buffer layer, a Zn(O,H) film, a Zn(O,S,OH) film, or the like formed by the solution method (see, for instance, Tokio Nakada et al., “Thin Solid Film” 431-432 (2003) 242-248).